BSNL has free data roaming in Delhi and Mumbai's MTNL network aswell and vice versa for Mtnl users going out of these metros.
But there are no unlimited plans and the 21mbps speed you are talking about is just on papers from what I know and the most you would get is 1.5mbps kind of speed provided you are at the right place at the right time
Your ID suggests you are a gamer and if online gaming is what you are planning to do with one of these 3g connections or even EVDO from bsnl, reliance or Tata then just forget abou tit and live with your 256kbps or 512 adsl connection and be happy for now. Things will change in future but we are just not there yet! and its a shame.